Behavior of Concrete Piles Confined with CFRP Grid

Transcription

1 Behavior o Concrete Piles Conined with CFRP Grid Lining Ding, Hatem. M. Seliem, Sami. H. Rizkalla, Gang Wu and Zhishen Wu Synopsis: This paper describes an experimental program undertaken to study the behavior and eectiveness o using Carbon Fiber Reinorced Polymer ( CFRP ) Grid, as an alternative or steel spirals to conine precast concrete piles. The research ocuses on the eectiveness o the coninement o the speciied C-Grid on the concrete core o piles. The experimental program consists o a total o seven short piles including one without coninement, two with steel spiral and our with C-Grid. The parameters included in the study were the number o grid layers, the overlap length, and the spacing between the circumerential wires o C-Grid. All the specimens were subjected to concentric axial compression up to ailure. Results indicate that the speciied C-Grid can provide equivalent perormance or more than typical spiral steel reinorcement or precast prestressed concrete piles. The paper also presents an analytical model to predict the perormance o piles reinorced with C-Grid as spiral reinorcement. The analytical model yields results that match well with the experimental results.. Keywords:Axial Compression, CFRP Grid, Concrete Piles

2 Lining Ding Doctoral Candidate o College o Civil Engineering, Southeast University, Nanjing, Jiangsu Province, the People s Republic o China. Visiting Scholar o Department o Civil, Construction and Environmental Engineering, North Carolina State University, Raleigh, NC, USA. Hatem.M. Seliem Assistant Proessor o Structural Engineering, Department o Civil Engineering, Helwan University, Cairo, Egypt. Sami.H. Rizkalla Distinguished proessor o Civil Engineering and Construction, director o the Constructed Facilities Laboratory, director o the NSF I/UCRC-CICI at North Carolina State University, Raleigh, North Carolina. USA. Gang Wu Dean and Proessor o College o Civil Engineering, Southeast University, Nanjing, Jiangsu Province, the People s Republic o China. Zhishen Wu Dean o International Institute or Urban Systems Engineering, Southeast University, Nanjing,Jiangsu Province, the People s Republic o China. Proessor o Department o Urban & Civil Engineering, Ibaraki University, Hitachi City, Ibaraki Preecture, Japan. 1. INTRODUCTION Fibre Reinorced Polymer (FRP) materials recently have become more popular as reinorcements or precast concrete structural elements, due to their non-corrosion characteristics and high strength-to-weight ratio [1],[2]. Speciically, Carbon Fibre Reinorced Polymer (CFRP) orthogonal Grid has been selectively used as an alternative reinorcement or several precast concrete elements [3],[5]. During the past ive years, the use o FRP grids as reinorcements o several precast concrete products has been investigated. Research indings indicated that FRP grid could be used as an alternative lexural reinorcement or slabs and beams as well as hoop reinorcement or coninement o concrete columns [2]-[8]. Recently, some urther research has been reported on the use o FRP grid or the precast concrete industry including double Tees and Sandwich wall panels [2]. For concrete columns, some researchers have studied the eectiveness o conining them using externally bonded FRP straps. Further, very little research has been reported on the use o FRP Grids or concrete piles [4], [7] and [8].

3 FRP grid products are classiied as heavy or light FRP grid based on the size o FRP strands. The strand width o the heavy FRP grid is typically ½ inch (12.7mm), while or light FRP grid is typically ¼ inch (6.4mm). CFRP Grid (C-Grid) is usually shaped into tubular shape and secured by plastic ties. The CFRP grid tube surrounds the longitudinal reinorcements to provide coninement to the concrete core o piles. In the structural applications o FRP Grid, CFRP light grid might be the proper selection to replace the steel spiral reinorcement due to the higher strength and better durability to harsh environment in comparison to Glass FRP grid, and less cost compared to CFRP heavy grid. This paper summarizes an experimental program, where seven short-length piles simulating the top end o piles in construction, were tested. The objective o this program is to investigate whether the speciied C-Grid is eective enough to provide adequate coninement to concrete core in replacement o the traditional steel spirals. The steel spirals are typically placed at the top end o piles to avoid premature ailure caused by the release o prestress and the impact loading during driving. The perormance o C-Grid was evaluated based on the measured ultimate strength o conined concrete, the axial strain at ultimate as well as ailure modes o piles under the eect o compression loads. An analytical model is proposed which yields results that match well with the experimental results. 2. RESEARCH SIGNIFICANCE The experimental study investigates the easibility o using C-Grid as replacement or steel spirals at the top end o precast prestressed concrete piles. The coninement improves load carrying capacity o concrete by providing lateral restrain. In addition, conining the top end o the piles prevents concrete crushing due to the release o prestress and driving orces experienced during construction. C-Grid can be a good alternative due to its non-corrosion nature and high strength-to-weight ratio with equivalent conine eectiveness compared to steel spiral. While the tests are perormed on pile specimens the research is also applicable to short concrete columns. 3. TYPE o C-GRID The C-Grid used in this investigation was produced by Chomarat North America o Anderson, SC. The selected C-Grid is designated by the manuacturer as C (C5500AX100). The grid has C50 (50 ksi or 345MPa) wires in both directions at a nominal spacing o 1.75 in. (44mm). Details o this C-Grid are shown in Figure 1 (a) and (b).

4 1.75" C 50 transverse wire C 50 main wire 1.75" (a) Spacing o wires (b) photograph o this type o C-Grid Figure 1-- Coniguration o C-Grid A total o twenty-our tension coupons were cut rom both grid directions and tested using a universal testing machine and a 2-in. (51mm) extensometer. The coupons were prepared by applying taps on each end. The taps were made o our layers o GFRP abric to enable gripping the coupon as shown in Figure 2. The total length o each coupon was 14 in. (356mm) including 4 in. (102mm) o GFRP tabs at each end. Figure 2--Tensile test o C-Grid coupon The measured ultimate tensile loads and strains or the tested coupons in both directions are given in Table 1. Table 1 Material properties o C-Grid wires Average Max Load or Main Wires, lb(n) 1139 (5066) Average Max Load or Transverse Wires, lb (N) 1175 (5226) Average Ultimate Tensile Strain or Main Wires, ε Average Ultimate Tensile Strain or Transverse Wires, ε EXPERIMENTAL PROGRAM The experimental program consists o seven pile specimens subjected to concentric axial compression up to ailure. The specimen represents a 3 t long top section o a precast concrete pile. The detailed dimensions and cross sections o the specimens with steel spiral or C-Grid are shown in Figure 3. The specimens had a 14 in. (356mm) square cross-section

5 6" 3-9" 2-9" 6" with an overall height o 45 in. (1143mm). Steel collars were provided at each end o the specimens to prevent premature local crushing o concrete. Four 6 in. (152mm) height and 15 in. (381mm) wide steel plates were welded together to make this 1/2 in. (13mm) thick steel collar. The thicknesses o concrete cover were selected 3 in. (76mm) or specimens conined with steel spirals and 3/4 in. (19mm) or specimens conined with C-Grid. Steel Spiral Coninement Provided w/3" Clear Clear Cover Cover (Coninement Varies, See See Table Table 1) 1) Steel Steel Collar Collar with with 1 2" 1 Wall 2" Wall (a) Cross section o specimens conined with steel spiral Grid C Grid Coninement Provided w/ 4" 3 Clear Cover Coninement Provided w/ (Coninement Varies, See 4" 3 Clear Cover Table 1) (Coninement Varies, See Table 1) Steel Collar with 1 2" Wall Steel Collar with 1 2" Wall (b) Cross section o specimens coninement with C-Grid 14" Square Concrete Pile Steel Collar (c)coniguration o specimens Figure 3 Details o test specimens Seven specimens were included. One control specimen, CN, was without longitudinal and transverse reinorcement. Two specimens, S@6 and S@3, were conined with W3.5 square spirals at a spacing o 6 in. (152 mm) and 3 in. (76 mm), respectively. Two

6 specimens, 1C-8 and 1C-16, were conined with one layer o C-Grid with 8 in. (203 mm) and 16 in. (406 mm) overlap, respectively. One specimen, 2C-C-8, was conined with two continuous layers o C-Grid with 8 in. (203 mm) overlap. One specimen, 2S-C-8, was conined with two separate layers o C-Grid with 8 in. (203 mm) overlap. Table 2 gives the details o the test specimens. The concrete used was ready mix concrete provided by local supplier. All the concrete cylinders were cast out o the same batch o concrete. They were tested on the same testing day o concrete piles. The average compressive strength o concrete was 5560 psi (38MPa). Table 2 Reinorcing details o specimens Specimen Details o Coninement Longitudinal Reinorcement CN No Coninement None S@6 W3.5 Square Steel 6 in. (152mm) 4 NO. 3 (G60) S@3 W3.5 Square Steel 3 in. (76mm) 4 NO. 3 (G60) 1C-8 1 Layer o C-GRID with 8 in. (203mm) overlap None 1C-16 1 Layer o C-GRID with 16 in. (406mm) overlap None 2C-C-8 2C-S-8 2 Continuous Layers o C-GRID with 8 in. (203mm).overlap 2 Separate Layers o C-GRID with 8 in. (203mm) overlap None None It should be noted that our No.3 longitudinal bars were used in specimens S@6 and S@3 to maintain the pitch o steel spirals, however no longitudinal bars were used in other specimens. C-Grid was ormed into tubular coniguration and secured with plastic ties, and placed into orm to provide a clear concrete cover o ¾ in. (19 mm), as shown in Figure 3(b). The concrete cover provided or the C-Grid was less than that or the steel spiral since the material is non-corrosion. The two separate layers o C-Grid or specimen 2C-S-8 were assembled together and staggered to locate the circumerential wires into hal the spacing o 2C-C-8, as shown in Figure 4. All the specimens were tested using a 2000-kip (9000 kn) axial compression machine. Two string potentiometers were used to measure the axial shortening. Two concrete strain gauges and two PI gauges were placed at the middle height o piles to measure the strains o the concrete in both axial and transverse directions. In addition, electrical resistance strain gauges were attached to the steel spirals and the C-Grid beore casting to measure their strains. The test setup used in the experimental program is shown in Figure 5.

7 8" Overlap 8" Overlap 8" Overlap 8" Overlap CGRID CGRID layer #2 layer #2 2 layers 2 layers o continuous o continuous CGRID CGRID with 8" with o overlap 8" o overlap 8" Overlap 8" Overlap 2.36" 2.36" 1.18" 1.18" P.C2.a P.C2.a P.C2.b P.C2.b (a) 2C-C-8 (b) 2C-S-8 Figure 4 C-Grid details or specimen 2C-C-8 and 2C-S-8 Figure 5 Test setup

8 5. TEST RESULTS Test results, including the measured maximum applied load, concrete compressive strength, axial shortening at maximum load, and measured concrete axial strain at maximum load are given in Table 3. Comparing the ultimate load o the control specimen to those conined with C-Grid, result indicates that constraining the lateral dilation o concrete by the use o C-Grid develops a tri-axial stress state within concrete core, and urthermore leads to the enhancement o the axial load-carrying capacity. Table 3 Summary o test results Specimen ID Peak Load kips (kn) Conined Concrete Strength ksi (MPa) Axial Shortening at Peak Load In. (mm) Axial Strain at Peak Load με CN 619 (2753) 5.56 (38) (1.32) 1808 S@6 769 (3421) (83) (2.36) 3774 S@ (4679) (113) (2.21) C (4679) 8.58 (59) (2.95) C (4435) 8.11 (56) (3.18) C-C (4270) 7.81 (54) (2.64) C-S (3727) 6.82 (47) (2.44) 1499 The load-axial shortening relationships or all tested specimens are compared in Figure 6. During the loading o the control specimen CN, the steel collar ailed which led to local ailure at the two ends o specimen. At ailures, it was observed that once the concrete cover spall o, C-Grid lost its anchorage provided by the overlap, and the wires ruptured due to the induced high stress concentration developed in the overlap region. Test results shown in Figure 6 also indicate that the coninement provided by one layer o C-Grid has the same eect on axial load carrying capacity as that o steel spirals or specimen S@3. In addition, increasing the overlap length rom 8 in. (203 mm), 1C-8, to 16 in. (406 mm), 1C-16, did not inluence the ultimate load carrying capacity o pile. The results also indicate the use o 8 in. (203 mm) overlap is suicient to develop the ull strength o the C-Grid. Furthermore, the results show that two continuous layers o C-Grid are more eicient than the two separate layers as indicated by the higher load capacity. This behaviour might be attributed to the vertical shit between the two separate layers causing the reduction in the spacing between adjacent circumerential wires.

9 Figure 6 Load-axial shortening behaviour o specimens The axial compressive loads versus axial strains relationship or all tested specimens are shown in Figure 7. It was observed that the eective cross section areas o specimens were reduced during loading processes due to the gradual degradation o cover concrete. However it is very diicult to calculate the cross section area at any loading moment. Thus compression loads are plotted out in Figure 7 instead o compressive stress. And it should be noted that axial strains are considered negative. Test results indicate that all C-Grid conining specimens, except or the specimen with two separate layers (2C-S-8), perormed equivalent to that o S@3 including the maximum load and strain. This indicates that C-Grid can be used as a replacement to steel spirals to resist the impact o driving loads at the ends o precast concrete piles. All tested specimens ailed in a brittle mode. Initially longitudinal cracks were developed on the outer surace and then expanded to core concrete area very quickly aompanied with a series o rupture sound o the FRP wires. Failure ourred ater the majority o the concrete cover spall o,as shown in Figure 8 or the typical specimen 2C-C-8.

10 Figure 7 Axial strain behavior o tested specimens (d) 1C-8 () 2C-C-8 Figure 8 Shape o the specimen at maximum load

11 Ater testing, the concrete cover was careully removed. And it was ound that the observed longitudinal cracks extended through the ull cross-section depth, indicating ailure o the concrete core and rupture o the C-Grid, as shown in Figure 9. Figure 9 Failure o specimens 6. NUMERICAL ANALYSIS OF CONFINEMENT USING C-GRID REINFORCEMENT This section provides design method or the use o C-Grid or conining concrete piles based on the research conducted by Mohamed Saai [3], Li, G [4] and [7], Michael, A.P. [5], Wu, G.[6] and Ji, G.[8]. The proposed model estimates the conined concrete compressive strength, terms o the unconined concrete compressive strength, as ollows. in co 1 k1 (1) l co where,, is the lateral conining pressure applied to concrete core as shown in Figure 10, l and k 1 is the coninement eectiveness coeicient based on the conining device.

12 Figure 10 Conining action o C-Grid For C-Grid conined concrete columns, the circumerential wires are spaced at certain intervals instead o continuously located as in the case o FRP sheets. Aordingly, can be calculated as ollows: 2 rp Arp 2 rp (2) l ds ds where, is the cross-section area o the circumerential wires o the C-Grid, is the maximum tensile load o one circumerential wire, d is the diameter o the core concrete, and s is the spacing between the transverse wires o C-Grid. The compressive strength o the conined concrete ( ) can be calculated as ollows, P A max (3) core where, is the measured maximum axial load o the test specimen and is the cross-section area o concrete core. Some researchers have tried to ind the corresponding conining coeicient Equation (1) as shown in Table 4. in Table 4 Various equations or the calculation o Karbhari and Samaan et Miyauchi Source Gao(1997):I al.(1998) et al.(1999) Equation Saai et al.(1999) Toutanji (1999)

13 Based on the research in Table 4, Lam and Teng (2002a) [9] established a database including all their test results o concrete columns conined with FRP sheets or FRP tubes, and then proposed a value o ater a series o careul analysis. This conined concrete strength model was proved more aurate than the previous ones. In order to evaluate the concrete strength o columns conined with FRP grids, Saai, M. [3] proposed the ollowing equation to calculate, l ( ) co k (4) Considering dierent FRP materials, dierent coninement ratios and the actual measured maximum loads, the validity o the two models is discussed by comparing the ratio o (experimental) to (predicted) as given in the last two columns o Table 5. Results rom reerences [3], [5], [6] are also included. The analysis indicates the two models overestimate and in some other cases under estimate the measured values or all the specimens. It is noticed that the predictive results o Saai M. s Model [3] are generally more conservative than those o Lam and Teng s Model (2002a) [9]. It is also observed that Saai M. s Model [3] has higher auracy than Lam and Teng s Model (2002a) [9] when applied to specimens having strain sotening response in compressive stress-strain relationship due to the small amount o FRP coninement, as the last ten specimens given in Table 5. Lam and Teng s Model (2002a) [9] behaves much better than Saai M. s Model [3] or the C-Grid conined concrete piles tested in this experimental program, although neither model provide very good auracy, as the irst our specimens given in Table 5. Thereore the relationships o versus co or all the tested C-Grid conined specimens shown in Figure 11 were used to drive an expression or the coninement coeicient k 1 as given in Figure 12. l co

14 Table 5 Comparison o the experimental and calculated values Sp«irudI ID Unconin Calculated Strdlgth Ie; ~lll(1ltal Ratioo -, " Ie; hi (Mpa) ConCl~ " SUdlgth " S tr~gth 1L Ie~ 50=, ~ ~ l=,rui Lrun """ Saai, Tmgs Saai, Tdlgs h i (Mpa) M..[J] ",odd M Y ] ",odd ksi (Mpa) (200la l (200la)[9) 1C-8 0, (59) 0,67 0,75 5,77 (40) 6.45 (44) IC-16 0, (56) 0, R.,,;.,arch 5.56 (38) program in 2C-C-8 0, (54) 0,76 0,88 lhi ~ pape1" 5.97 (41) 6.87 (47) 2C-S-8 0, (47) Ci Q3 (83) C (34) (66) JO.l 5 1J.75 0,84 0,98 (70) (81) R..,.,.-.:K;., , (64) (70) [J[ CJ (57) 8.56 (59) 9.28 (64) Grid I 0, (59) 0,89 1,05 Grid 2 0, (47) 7.63 (53) 8.92 (62) 1, Grid 3 0, (57) 0,93 1,08 R..,er~c., 6.84 (47) Grid 4 0, (50) 1, [51 Grid 5 0, (52) 7.61 (52) 8.88 (61) 101 U8 Grid 6 0, (54) 0, CRJ-30 0, (28) 4.35 (30) 5.22 (36) 107 1,29 CRJ-SO 0, (27) 4.06 (28) 4.79 (33) 104 },22 R..,ermc., 3.63 (25) CRJ-60 0,055 l.77 (26) 4.06 (28) 4.64 (32) 1,08 1,23 [6[ CRJ-30 (2 layers) (37) 5.22 (36) 5.95 (41) 0,97 U I

15 k1 Figure 11 Relationship o versus co l co used in Figure 11 is calculated by substituting the measured strain o the C-Grid circumerential wires into Equation (2) to obtain the stress in the FRP ( ). Figure 11 shows that the curves o 1C-8, 1C-16, and 2C-C-8 are similar and close to each other while the curve or 2C-S-8 is dierent. As the dierent coninement unction o two separate layers rom two continuous layers, hoop reinorcement strains o specimen 1C-8, 1C-16 and 2C-C-8 are selected to investigate the equation or k 1, as shown in Figure y = x l/co Figure 12-- Coninement Eectiveness Coeicient or C-Grid Conined Concrete

16 Figure 12 shows an approximate exponential relationship between the value o aording to Equation (1). The simulation results are given as ollows. The decimal precision is reduced to keep the same degree o auracy as Equation (4). k and l ( ) (5) co Using the proposed expression k 1 given in Equation (5) to predict the conined compressive strength provides good estimation as given in Table 6. Table 6 Calculation results Specimen ID Experimental Strength ksi (Mpa) Calculated strength ksi (Mpa) Ratio o 1C (59) 9.04 (62) C (56) 9.04 (62) C-C (54) 9.38 (65) CONCLUSIONS This paper summarizes test results o an experimental program under taken to study the eectiveness o C-Grid in replacement o steel spiral to provide coninement o the concrete or pile application. Summary o the research indings is: 1. C-Grid can be used eectively in replacement o steel spiral to provide coninement o the concrete or pile application. 2. Use o two continuous layers o FRP grid is more eicient that the use o two separate layers. 3. The conining model proposed by Lam and Teng (2002a) or predicting the ultimate strength o concrete strengthened with externally bonded FRP sheets, may be extended to predict the strength o concrete piles conined internally by CFRP grid. 4. The proposed conining coeicient (k 1 ) model based on the limited specimens tested in the experimental program matches reasonably well with the test results and posses a high potential or predicting the ultimate strength o FRP grid conined concrete. However, urther testing is required to evaluate its auracy.

17 8. REFERENCES [1] Fam, A. and Rizkalla, S., "Coninement Model or Axially Loaded Concrete Conined by Circular Fiber Reinorced Tubes", ACI Structural Journal, Vol 98, No. 4, July-August [2] Dawood, M. and Rizkalla, S., 3-D Pultruded GFRP Sandwich Panels or Civil Engineering Inrastructure and Transportation, Submitted to ASCE Composite or Construction, September 10, [3] Saai, M., Design and abrication o FRP grids or aerospace and civil engineering applications. Journal o Aerospace Engineering, Vol.13, No.4: [4] Li, G.,2005. Experimental study o hybrid composite cylinders. Composite Structures, 78 (2007) [5] Michael, A.P., Using Carbon Fiber Reinorced Polymer Grids as Coninement Reinorcement or Concrete. Doctoral thesis, University o Florida. [6] Wu, G., A seismic perormance o circular concrete columns conined with FRP grids. Journal o Architecture and Civil Engineering, Vol.24, No.5: [7] Li, G., Fabricating, Testing, and Modeling o Advanced Grid Stiened Fiber Reinorced Polymer Tubes Encased Concrete Cylinders. Composite Materials, Vol.42, No.11: [8] Ji, G., Ouyang, Z., Experimental investigation into the interacial shear strength o AGS-FRP tube conined concrete pile. Engineering Structures, 31 (2009) [9] Teng, J.G., FRP Strengthened RC structures. John Willey & Sons, Ltd. Page 245.